1. Technical Field
The invention relates to a driving system for an electro-luminescence display device and more particularly, to a driving system for an electro-luminescence display device having an organic light emitting diode.
2. Related Art
A flat panel display device includes a liquid crystal display device, a field emission display device, a plasma display device, an electro-luminescence (EL) display device, etc. The EL display device is a self-light emitting device for emitting a fluorescent material by re-combining electrons and holes. The EL display device may be divided into an inorganic EL device which uses an inorganic compound as the fluorescent material and an organic EL device which uses an organic compound as the fluorescent material.
The EL display device may be driven at a low driving voltage 10V and has excellent recognition characteristics due to the self-light emitting. The EL display device may be thin because no backlight is needed. The EL display device may have advantages over LCD, such as a wide viewing angle, a quick response speed, etc.
The organic EL display device includes an electron injection layer, an electron transport layer, a light emitting layer, a hole transport layer and a hole injection layer, which are laminated between a cathode and an anode. In the organic EL device, when a certain voltage is applied between the anode and the cathode, electrons generated from the cathode move toward the light emitting layer through the electron injection layer and the electron transport layer. Holes move toward the light emitting layer through the hole injection layer and the hole transport layer. The electrons and the holes, which are supplied from the electron transport layer and the hole transport layer, are recombined in the light emitting layer, thereby emitting light.
The EL display device includes an organic light emitting diode (OLED) panel that a plurality of pixels is arranged in a matrix. Pixels include an EL cell such as an OLED. The OLED panel is connected to a scan driver and a data driver, which are controlled by a controller. The scan driver operates to activate a pixel and the data driver provides a driving voltage to the activated pixel. The pixel emits light in response to the driving voltage. Each pixel represents one of red (R), green (G) and blue (B) colors.
The EL display device may display an image in a gray scale. In the EL display device, each pixel is controlled to emit light or light off per frame. More specifically, each frame is divided into multiple sub-frames and the pixel emits light or lights off during the sub-frames in response to each bit of a digital data signal. For example, for a 12 bit digital data signal, a frame is divided into 12 sub-frames. The light emitting time of the pixel during each sub-frame is summed and represents a desired gray scale of an image.
For a gray scale display, a digital data is converted into another digital data based on a look up table (“LUT”). The LUT is stored in a controller that drives the scan driver and the data driver of the EL display device. The digital data signal is input to the controller. The controller may have a multiplexer that receives the digital data signal and determines that the digital data signal corresponds to a red (R) signal, a green (G) signal or a blue (B) signal. The controller may include three separate LUTs that are used with the R signal, the G signal and the B signal, respectively.
FIG. 1 illustrates three LUTs for use with the R, G and B data signals. Each LUT has a plurality of index values that corresponds to different digital data signals. As shown in FIG. 1, LUT-R, LUT-G and LUT-B have different index values in response to different digital data signals. For example, when a 6 bit digital data signal is 111110, LUT-R has an index value of 11111111, LUT-G has an index value of 10111111 and LUT-B has an index value of 11011111. The LUTs may not only convert the value of the digital data signal but also convert a bit number of the digital data signal. Specifically, when a 6 bit digital data signal is input to the controller and processed through the LUT, an 8 bit digital data signal having a different bit stream is output from the controller. This 8 bit digital data signal is supplied to the data driver. The bit number of the digital data signal is expanded to perform a gamma control and display a desired gray scale.
The EL display device may use the different LUTs for the R, G and B signals to achieve color coordinates, a gamma control and a contrast ratio. In the OLED panel, color pixels such as a red (R) pixel, a green (G) pixel and a blue (B) pixel may have a different efficiency. The different LUTs having different index values for the R, G and B signals may compensate for the difference in the R, G and B pixels. Upon application of the same source voltage VDD, however, the R pixel, the G pixel and the B pixel may not represent a desired gray scale image. When the same source voltage is applied to a driving transistor of the R pixel, the G pixel and the B pixel, a different color response may develop in R, G and B pixels. The source voltage VDD and the LUTs may be predetermined and uncontrollable once the EL display device is in operation.
Further, the EL display device displays an image with a full white brightness level, regardless of an ambient environment. As noted above, the source voltage VDD is preset and may not be changed in response to the ambient environment. Power consumption may increase. Therefore, there is a need of a driving system for an EL display device that obviates drawbacks of a driving method of the related art EL display device.